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If sight depends upon the linking of the light of vision with
the light leading progressively to the illumined object, then, by
the very hypothesis, one intervening substance, the light, is
indispensable: but if the illuminated body, which is the object
of vision, serves as an agent operating certain changes, some
such change might very well impinge immediately upon the eye,
requiring no medium; this all the more, since as things are the
intervening substance, which actually does exist, is in some
degree changed at the point of contact with the eye [and so
cannot be in itself a requisite to vision].
Those who have made vision a forth-going act [and not an
in-coming from the object] need not postulate an intervening
substance- unless, indeed, to provide against the ray from the
eye failing on its path- but this is a ray of light and light
flies straight. Those who make vision depend upon resistance are
obliged to postulate an intervening substance.
The champions of the image, with its transit through a void, are
seeking the way of least resistance; but since the entire absence
of intervenient gives a still easier path they will not oppose
that hypothesis.
So, too, those that explain vision by sympathy must recognize
that an intervening substance will be a hindrance as tending to
check or block or enfeeble that sympathy; this theory,
especially, requires the admission that any intervenient, and
particularly one of kindred nature, must blunt the perception by
itself absorbing part of the activity. Apply fire to a body
continuous through and through, and no doubt the core will be
less affected than the surface: but where we are dealing with the
sympathetic parts of one living being, there will scarcely be
less sensation because of the intervening substance, or, if there
should be, the degree of sensation will still be proportionate to
the nature of the separate part, with the intervenient acting
merely as a certain limitation; this, though, will not be the
case where the element introduced is of a kind to overleap the
bridge.
But this is saying that the sympathetic quality of the universe
depends upon its being one living thing, and that our amenability
to experience depends upon our belonging integrally to that
unity; would it not follow that continuity is a condition of any
perception of a remote object?
The explanation is that continuity and its concomitant, the
bridging substance, come into play because a living being must be
a continuous thing, but that, none the less, the receiving of
impression is not an essentially necessary result of continuity;
if it were, everything would receive such impression from
everything else, and if thing is affected by thing in various
separate orders, there can be no further question of any
universal need of intervening substance.
Why it should be especially requisite in the act of seeing would
have to be explained: in general, an object passing through the
air does not affect it beyond dividing it; when a stone falls,
the air simply yields; nor is it reasonable to explain the
natural direction of movement by resistance; to do so would bring
us to the absurdity that resistance accounts for the upward
movement of fire, which on the contrary, overcomes the resistance
of the air by its own essentially quick energy. If we are told
that the resistance is brought more swiftly into play by the very
swiftness of the ascending body, that would be a mere accidental
circumstance, not a cause of the upward motion: in trees the
upthrust from the root depends on no such external propulsion;
we, too, in our movements cleave the air and are in no wise
forwarded by its resistance; it simply flows in from behind to
fill the void we make.
If the severance of the air by such bodies leaves it unaffected,
why must there be any severance before the images of sight can
reach us?
And, further, once we reject the theory that these images reach
us by way of some outstreaming from the objects seen, there is no
reason to think of the air being affected and passing on to us,
in a progression of impression, what has been impressed upon
itself.
If our perception is to depend upon previous impressions made
upon the air, then we have no direct knowledge of the object of
vision, but know it only as through an intermediary, in the same
way as we are aware of warmth where it is not the distant fire
itself that warms us, but the warmed intervening air. That is a
matter of contact; but sight is not produced by contact: the
application of an object to the eye would not produce sight; what
is required is the illumination of the intervening medium; for
the air in itself is a dark substance: If it were not for this
dark substance there would probably be no reason for the
existence of light: the dark intervening matter is a barrier, and
vision requires that it be overcome by light. Perhaps also the
reason why an object brought close to the eye cannot be seen is
that it confronts us with a double obscuration, its own and that
of the air.
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